1. Field of the Invention
This invention relates generally to a digital audio radio system and, more particularly, to a digital audio radio system that incorporates a technique for eliminating or minimizing the effects of a blocked transmission without the need for additional system resources.
2. Discussion of the Related Art
State of the art technology for current radio systems includes a digital audio radio system (DARS) that generates compressed digital audio signals to be transmitted by a digital audio transmission source and reproduced in a receiver of the DARS. In the known DARS, the audio signals are generated in a broadcast studio and converted to digital data. The digital data signals are then sent to an earth based ground transmission station to be transmitted to a plurality of receivers within a reception area or to be transmitted to one or more satellites orbiting the earth in a geosynchronous orbit. The satellites then transmit the radio signals to a defined reception area over the earth.
The broadcast studio generates analog audio signals in much the same way as a conventional radio station studio. For example, an announcer provides real-time narration, and then typically plays music selections from a library of recorded music, such as compact disc (CD) music albums. The analog signals of the narrations are converted to a digital data stream using pulse code modulation (PCM). PCM is sometimes referred to as uncompressed, raw digital audio, and is a continuing, in time, sequence of regularly spaced (temporal period) digital words, for example, 16 bits of resolution for a total of 65, 536 amplitude levels. The value of each sample represents the quantized audio signal level (amplitude) at that sample's instant in time. The conversion is performed for a real-time voice or live music performances by passing the analog signals through an analog-to-digital converter (ADC). In the case of playing music CD, the analog-to-digital conversion is not necessary since the audio data is already stored in a digital format on the CD.
The PCM digital audio signal has a data rate of 44,100 samples per second (sps), 16 bit linear quantization, and stereo left and right channels. This is the data stream quality of CD music in temporal sampling resolution and amplitude resolution. This PCM digital data is then compressed by, for example, a psycho-acoustic coding (PAC) compression algorithm into a 128 kilo-bits-per second (kbps) digital signal to conserve system data throughput resources. The present ability of audio compression algorithms and the listener community consensus is that PAC compressed 128 kbps stereo is imperceptibly different than a 44,100 sps PCM signal. PAC is a type of audio compressor algorithm which preserves only those sounds important to the human ear-brain connection, and is thus efficient. In accordance with these known compression algorithms, the original audio wave by itself is not preserved and replicated, rather, key mathematical features of the wave form, such as frequency bin activity, comprise the compressed data bit stream. The compressed audio data of PAC consists of successive frames of data, each on the order of 1/30 of a second of sound. Organizations such as Dolby, MPEG, AT&T, Musicam and others have various versions of PAC algorithms, implemented in either software or hardware. A compressed data rate of 128 kbps would produce essentially the same audio quality as a music CD. For purposes of the discussion below, 128 kbps will be referred to as 100% data throughput.
For satellite systems, the digital data is sent to the earth based ground station for transmission to one or more satellites on a radio frequency "uplink" carrier. The digital data is generally compressed in the ground station. The satellite receives the signal from the ground station and then retransmits it to a defined area on the earth's surfaces where radio reception is desired. For example, the satellite can have a "downlink" beam pattern that covers the continental United States. The receiver receives the downlink signal, decompresses it and converts it back to an analog signal for both stereo channels using a digital-to-analog converter (DAC) for subsequent amplification and listening through speakers.
In the satellite based systems, certain unique problems exist if the receiver is mobile, such as a car radio. There are many sources of line-of-sight (LOS) obstructions as one drives along a typical road or highway. For example, trees attenuate the downlink signal from the satellite, and can even render it unusable if the foliage is particularly dense. Additionally, there are other sources of complete blockage of the downlink signal, such as billboards, buildings, bridge overpasses, large trucks, etc. The obstructions can be divided into two groups, those that attenuate the downlink signal, and those that completely block the downlink signal. To alleviate attenuation problems, extra signal strength can be used to "burn through" the obstruction. Additionally, schemes such as interleaving redundant digital bits so that occasional signal bit errors have an imperceptible effect, or an error-detection/correction coding can be used. As long as there is adequate signal, and adequate error correction, radio performance is generally unaffected.
For those types of objects that provide total blockage of the signal, other techniques to mitigate the blockage are required. One technique is to employ two or more identical satellites, broadcasting the same radio program material. The satellites would be positioned in their orbits so that the angle from each satellite to the user's radio would be substantially different. Thus, the chances of both signals being obscured at the same time are unlikely, and the radio operation would continue without interruption. Another approach is to provide two antennas mounted on a vehicle, one on the hood, and one on the trunk lid. Reception loss from smaller obstructions would thus be prevented because at least one antennae would be in view of a satellite as the vehicle travels.
To eliminate audio reception outages in moving vehicles when the downlink signal from the satellites is intermittently blocked by objects as the vehicle moves, it has been known to provide a time-shifted pretransmission of the signal stored in the receiver's memory to be substituted for the signal later if it is blocked. U.S. Pat. No. 5,592,471 issued Jan. 7, 1997 to Briskman, discloses a mobile radio receiver using time diversity to avoid service outages in multi-channel broadcast transmission systems. This invention uses a "substantial identical" signal pretransmission method. A digital audio signal is generated as discussed above, and split into identical first and second transmission "legs." Prior to transmission, one of the legs is delayed a predetermined time. The two legs are transmitted to one or more satellites to be retransmitted and received by a receiver. The non-delayed leg is delayed in the receiver to coincide with the delayed leg. Thus, two identical signals are received in the receiver spaced apart in time. If the signal currently being received gets blocked, the receiver can select the delayed leg already present in the receiver that was received before the signal was blocked. If a single pretransmission leg is employed, the total system data rate would be 256 kbps, or a 200% throughput because of the redundant 128 kbps transmission leg. U.S. patent application Ser. No. 08/665,143, titled "Method And Apparatus For Accommodating Signal Blockage In Satellite Mobile Radio Systems", filed Jun. 14, 1996, assigned to the assignee of the instant invention, and herein incorporated by reference, also discloses a technique for preventing loss of signal due to blockage in a satellite DARS. This system also incorporates a pretransmission technique where a "present" data stream and a "future" data stream are time multiplexed for transmission. A buffer is employed in a pretransmitter that stores consecutive frames of digital audio signals to be transmitted in a shift register type configuration. The pretransmitter selects a future frame from an input end of the buffer and combines it with a present frame at an output end of the buffer to be transmitted as a single "leg" of the audio transmission signal. Each time the shifter register buffer shifts a frame, a new leg is generated as part of the data stream that is to be broadcast. A satellite receives the digital data stream and retransmits it over the defined area to be received by a receiver. The receiver also includes a shift register buffer that inputs the future portion of each leg at the input of the buffer. As the future portion of the received signal is shifted through the register, the future portion of the leg is the same in time as the present portion when it reaches the output of the buffer. An OR-gate selects one of either the present portion of each leg of the received signal or the future portion at the output of the buffer as the current transmission. Thus, if the signal from the satellite is blocked, and no present signal is available at the antenna of the receiver, the future portion stored in the shift register may have been stored prior to when the blockage occurred and can be used. Additionally, an intermediate leg (soon leg) can be transmitted between the future and present portions and stored in the middle of the buffer to accommodate brief occurring obstructions.
Storing pretransmitted legs to be used in the event of signal blockage is referred to in application Ser. No. 08/665,143 as a PSF (present, soon, future) pretransmission technique. To provide PSF legs in a practical implementation having an acceptable data rate overhead, the pretransmitted legs (soon, future) can be of a lesser data quality (music replication accuracy) to save system resources (bandwidth), if the pretransmitted audio quality is user-acceptable. The invention of application Ser. No. 08/665,143 contemplates any technique in which multiple time-separated versions of the same source signal can be simultaneously transmitted and independently separated by a receiver where one or more of the versions has a lesser data quality than the version used when the signal is not blocked.
The more transmitted legs there are, the more robust the system is in accommodating physical blockage or other signal fading situations. However, the greater the digital data rate transmitted to and from the satellite, the bigger the satellite needs to be and more power is required to operate it. Thus, it is of paramount importance to conserve transmitted data rate to the extent possible. If the technique is employed where pretransmitted audio is of a lesser data rate acceptable sound quality, then the present and future pretransmitted legs might be accomplished with a 128 ksps stereo present signal, plus a 64 kbps stereo future signal. 64 kbps stereo is virtually indistinguishable in audio quality from 128 kbps stereo, except in the most ideal of listening conditions, by the most critical of listeners and is thus acceptable for system use. This would result in a data rate throughput of 150% based on (128+64)/128%.
It would be ideal if providing multiple time-separated versions of a source signal did not have an increase in bit data rate (bandwidth) over the nominal data rate needed for an acceptable audio reception. It is an object of the present invention to provide a DARS that provides a technique for providing suitable audio in the event that there is signal blockage to provide maximum pretransmitted legs while using no or little additional system resources.